static bool scan_up_end_bit(int bit) { return bit == (1 << pcu_floor_log2(pcu_mpi_size())); }
static int scan_down_begin_bit(void) { return 1 << pcu_floor_log2(pcu_mpi_size()); }
int pcu_ceil_log2(int n) { int r = pcu_floor_log2(n); if ((1 << r)<n) ++r; return r; }